Atomic emission spectroscopic detectors for gas chromatography have become an important analytical methodology because they simplify the interpretation of complex chromatograms by providing element specific information about each eluting peak. Plasma sources play an important role in this field. A. J. McCormack, S. C. Tong, and W. D. Cooke, published an article Anal. Chem., 1965, 37, 1470, based on plasma emission spectroscopy. Since that time, several kinds of plasmas, including microwave induced plasmas (MIP), direct current plasmas (DCP), alternating current plasmas (ACP), and inductively coupled plasmas (ICP) have been utilized as gas chromatography detectors. The MIP has been the most successful because a relatively low power is required to sustain the plasma, a relatively small quantity of gas is consumed during routine operation and the detector volume can be kept small.
In initial work using a MIP as a gas chromatography detector, McCormach et al. used a 2450 MHz, atmospheric pressure argon discharge to detect the elution of halogen and other non-metal containing compounds. The detection limits for most of the non-metal elements were found to be in the range of 10.sup.-12 to 10.sup.-9 g/s. C. A. Bache and D. J. Lisk, Anal. Chem., 1965, 37, 1477; C. A. Bache and D. J. Lisk, Anal. Chem., 1966, 38, 783; C. A. Bache and D. J. Lisk, Anal. Chem., 1966, 38, 1757 and C. A. Bache and D. J. Lisk, Anal. Chem., 1967, 39, 786 used a helium MIP at low pressure (5-10 mm Hg) and found that better power coupling and hence better atomization characteristics could be obtained. This situation was improved with the development of the C.I.M. Beenakker, Spectrochim. Acta., Part B, 1976, 31B, 483 TM010 cylindrical resonance cavity which allowed efficient power coupling to an MIP at atmospheric pressure and at a relatively low power level (40-100 w).
Although the MIP is an excellent excitation source for element specific gas chromatography detection its use is constrained by some operational limitations. MIP cavities must be resonant with the driving frequency. According to C.I.M. Beenakker, and P.J.W.M. Boumans, Spectrochim. Acta., 1978, 33B, 56, coupling of power using a fixed loop as originally described by Beenakker is adequate for a helium plasma but not for an argon plasma. Additionally, discharge conditions in MIP depend both on the inner diameter of the cavity and the dielectrics inside the cavity. An MIP is normally ignited using a Tesla discharge.
U.K. Patent No. 2,183,087 A discloses a method and apparatus to produce a noble-gas plasma for excitation in optical emission spectrometry. The apparatus includes an hf generator 8 feeding operating at the resonant frequency of an oscillation circuit 1 consisting of at least one inductor L and one capacitor C1 . The capacitor includes at least two capacitor plates 10, 11 which are so shaped and mutually arranged that they enclose a cavity 12 in which the plasma may form. A sensor (22, FIG. 9) may detect the magnetic field and a regulating circuit 17 may adjust the capacitance of a second capacitor C2 in parallel with capacitor C2 to tune the oscillator circuit to the desired frequency, particularly during start up. This patent shows a electrode formed of a pair of facing semi-circular plates 12 (See FIGS. 1 and 2) which generate a short length plasma which thus has little sensitivity. No operating or analytical data is disclosed to confirm the viability and functionality of the method and apparatus.